Investigation of shallow to deep subsoil structures in urban area by means of microtremors and its application to an assessment for regional seismic input motions.

本文以日本北部的札幌市区为试验场，通过实验的方式，探索一种可行的评价城市地区地震输入运动的方法。本研究采用微震测量，因为它易于用于检测底土状况。首先，观测系统在硬件和软件方面都在不断升级。在笔记本电脑上安装A/D转换器，并通过计算机程序实现实时监控。利用该系统可以在繁忙的商业区进行白天测量，尽管信噪比很低。1990年和1991年在札幌市区进行了长周期微震的广泛测量。设置了与海岸线平行的7条观测线和与海岸线垂直的2条观测线，覆盖面积约40 × 40公里，共183个站点获得记录。在北海道大学设立了一个参考点，以监测测量期间微震的时间变化。通过将傅里叶光谱除以参考位点对应的光谱，得到每个位点的光谱比。在0.5秒、1秒、2秒和4秒周期的地图上绘制这些比率，并绘制4张等高线图作为地震微带图。利用少量深钻资料、三次强震记录和1982年Off-Urakawa地震的烈度分布资料对这些图进行了验证。

The purpose of this study is to develop a feasible method for evaluating seismic input motions in an urban area by means of experimental way and Sapporo urban area, northern Japan, was selected as a test field.Microtremors measurement was adapted for this study because of its easy applicability for detecting subsoil conditions. At the first, the observation system was upgrading in both terms of hardware and software. An A/D converter was installed in a laptop computer and a real time monitoring was introduced by computer program. With this system daytime measurements in busy business district can be performed in spite of low S/N ratio.Extensive measurements of long-period microtremors were conducted in Sapporo urban area in 1990 and 1991. Seven observation lines parallel to the coastal line and two lines perpendicular to these were set up, covering the area of about 40 x 40 kilometers, obtaining records at totally 183 sites. One reference site was set up at Hokkaido University to monitor the time variation of microtremors during the period of measurements.Spectral ratios were obtained for each site by dividing the Fourier spectra by the corresponding spectra at the reference site. These ratios were plotted on a map for each period of 0.5, 1, 2 and 4 second and four contour maps were drawn as seismic microzonig maps. These maps were verified by various of small number of deep drilling data, three strong motion records and seismic intensity distribution data of the 1982 Off-Urakawa earthquake.